The invention relates to a charge-coupled device comprising:
a. a semiconductor body having a charge transport channel defined at the surface for storing and stepwise transferring information in the form of discrete charge packets;
b. a row of clock electrodes above the charge transport channel for inducing a pattern of alternating potential wells and potential barriers in the charge transport channel;
c. first means for applying voltages to the clock electrodes, these means being such that during the storage of charge packets the charge-coupled device is operated as an m-phase device and during the transfer of charge packets is operated as an n-phase device, m and n both being natural numbers and m being larger than or equal to 1 and n being larger than m.
The invention further relates to a camera provided with such a device.
Such a charge-coupled device is distinguished from the conventional charge-coupled devices in that the information is stored with a higher density in the channel than it is transported therein. As will appear hereinafter, this property is of importance more particularly, though not exclusively, for image sensors.
In image sensors of, for example, the frame transfer type, in which the CCD acts as a sensor, as a transport channel and as a memory, it is usual, for example, in the case of 4-phase transport to operate the sensor action also as a 4-phase system. A photosensitive element or pixel then corresponds to four electrodes. In the storage section, one storage site also corresponds to four electrodes.
However, when the device is operated in the manner described above, it is possible while maintaining the electrode configuration, to store a charge packet in every second electrode. In comparison with a conventional 4-phase CCD, this means for the photo-sensitive section a doubling of the number of pixels per unit surface area.
In order to avoid that during transport charge packets are combined due to the high density, the frame of the charge packets is extended by first shifting only the charge packet closest to the output over a certain distance, and by then shifting this packet and simultaneously the next packet etc., in which event each time the distances between successive charge packets are made sufficiently large for the normal 4-phase transport. The inverse is the case in the storage section, in which the frame is compressed again, as a result of which the charge packets are stored again in every second electrode. Because of the resemblance to the extraction and compression of the bellows of an accordion, this device is designated as the "accordion sensor" in the lecture entitled "The Accordion Imager: an Ultra High Density Frame Transfer CCD" by A. J. P. Theuwissen et al (International Electron Devices Meeting Technical Digest, San Francisco, 1984, pages 40-43).
Besides clock voltage generators, a charge-coupled image sensor also comprises electronic control means indicating how and when the device is expected to operate. These electronic means indicate by way of example when an integration period begins and ends, when a frame of the photosensitive section has to be shifted into the storage section, when the frame has been shifted completely into the storage section, and so on. These electronic means essentially comprise a number of counters which count the pulses of a clock and give up a signal after a given number of pulses, as a result of which the device can pass from one condition to another.
An additional complication may then occur due to the "interlacing" mode of operation, in which the pixels in two successive frames are relatively shifted by a half vertical pitch. In this mode of operation, the numbers of clock pulses required for the transport of the two frames are generally not equal to each other. Due to their complexity, these electronic control means are mostly arranged externally, that is to say outside the chip of the sensor itself.